Query Post Execution Showplan event is a very useful event to find problematic queries and execution plans based on CPU usage or duration while analyzing performance issues.
It can increase the execution time of all queries by a fraction of a millisecond, irrespective of the total query cost, which means, the overhead of query_post_execution_showplan event is high for cheaper queries compared to expensive queries.
In all cases, one should enable the event on an ad hoc basis and with restrictive predicate to limit the number of events.
The below video demonstrates this with hands-on example. SQLTest Tool simulates the hands-on labs for you, no registration necessary. You can practice the online hands-on example while watching the video.